1. Field of the Invention
This invention relates to a carbonaceous negative electrode material for nonaqueous secondary batteries, a process for producing the negative electrode material, and a nonaqueous secondary battery containing the negative electrode material. More particularly, it relates to a carbonaceous material having a specific micropore size distribution and amount which is particularly suited as a negative electrode material of nonaqueous secondary batteries, a process for producing the same, and a nonaqueous secondary battery typified by a lithium ionsecondary battery, which is suitable for use in compact and light electronic equipment.
2. Description of the Related Art
The demand for rechargeable secondary batteries has been increasing in pursuit of reduction of size and weight and improvement of function of electronic equipment. Clean nonaqueous batteries, such as nickel-hydrogen or lithium batteries, have been attracting attention as batteries meeting the demand, taking the place of lead or nickel-cadmium batteries from the viewpoint of power saving and environmental conservation. In particular, lithium ion secondary batteries have been gaining weight from their lightness and high voltage and been put to practical use. Lithium ion secondary batteries developed in the early years used metallic lithium in the negative electrode, which turned out disadvantageous in that a dendrite grows from metallic lithium to cause an internal short-circuit. Then materials capable of intercalating and deintercalating lithium ion have been developed in place of metallic lithium. Along this line, carbonaceous materials, such as coke, and graphite materials, such as natural graphite, have been used mainly (see JP-A-2-90863, JP-A-1-221859, and JP-A-63-121257, the term "JP-A" as used herein means an "unexamined published Japanese patent application").
Of the carbonaceous materials and the graphite materials, the carboneous materials prepared by calcining at relatively low temperatures, for example 1500.degree. C. or lower, can be seen as promising as high-capacity and low-cost negative electrode materials. On the other hand, although carbonaceous materials are relatively inexpensive and safe and have a high charge capacity in the initial stage of use, their charge and discharge capacity in the later stage is about 2/3 of the theoretical capacity density of carbon-graphite materials (372 mAh/g). Batteries containing the conventional carbonaceous materials are not satisfactory in charge and discharge capacity and efficiency, leaving room for improvement.
The inventors of the present invention previously found that the high initial charge capacity of a carbonaceous material owes to its volatile matter, that the volatile matter which is released on heating is partly carbonized to interfere with the manifestation of the capacity of the carbonaceous material, and that the volatile matter can be removed efficiently by pulverizing the carbonaceous material followed by heating at a high temperature (calcination) thereby to suppress remaining of the carbonized volatile matter. These findings were filed for a patent (JP-A-8-287911).